Traditionally, semiconductor materials, such as silicon, have been used to implement memory circuits. Typically, the semiconductor materials are used in combination with dielectric and conductive materials to fashion transistors, capacitors, resistors, inductors and other basic circuit elements, which are arranged in various combinations to form memory cells and other components of memory devices.
Other types of materials are currently being investigated to replace semiconductor materials in memory devices and other electronics applications, due to the ongoing desire to produce electronic devices with greater information storage density, lower cost, higher speed, and/or other desirable characteristics. Such new materials may include organic molecular materials that can store information by taking on various oxidation states in response to applied signals. Such materials offer potentially higher component density, response speeds, and/or energy efficiency in memory applications.
A variety of approaches have been proposed for molecular memory devices. For example, a hybrid molecular-silicon transistor for use in memory devices and other applications has been proposed. In a hybrid transistor, applying a negative potential between the gate and drain of the molecular transistor may charge a layer of redox-active molecules therein; applying a higher potential may discharge the same molecular layer. Because these “charge-storage” molecules may have at least two possible states (charged or discharged), such a device can be used as a 1-bit memory cell. A logical “1” can be written by charging the redox-active molecules, also called programming. A logical “0” can be written by discharging the redox-active molecules, also called erasing. A logical “0” and/or “1” can be read by sensing a drain-to-source current, which is modulated by the charged state of the redox-active molecules. Programming and erasing typically involves transfer of electrons to and from the molecules, typically by electron tunneling.
In addition, ZettaRAM™ technology is an emerging technology that may replace conventional dynamic random-access memory (DRAM) and other types of memory in computer and/or other memory systems. In ZettaRAM™ technology, the conventional capacitor in a memory cell may be replaced with charge-storage molecules to form a hybrid molecular-silicon capacitor. The amount of charge stored in the capacitor is independent of write voltage. In other words, there is a predetermined threshold voltage above which the device stores a fixed amount of charge, and below which the device discharges the fixed amount of charge. A logical “1” may be written by charging the molecules at a voltage above the threshold voltage (i.e., programming), while a logical “0” may be written by discharging the molecules at a voltage below the threshold voltage (i.e., erasing). In addition, multiple threshold voltages may be provided to enable multi-bit storage. Further description of molecular memory devices can be found in U.S. Pat. No. 7,061,791 to Bocian et al., U.S. Pat. No. 6,212,093 to Lindsey, U.S. Pat. No. 6,272,038 to Clausen et al., U.S. Pat. No. 6,944,047 to Rotenberg et al., U.S. patent application Ser. No. 11/266,776 to Bocian et al., and U.S. patent application Ser. No. 11/118,043 to Mobley et al., the contents of which are incorporated by reference in their entirety.
Hybrid molecular-silicon technology may be important not only in utilizing the advantages of the individual charge-storage molecules, such as discrete energy states, low voltage operation, nano-scale size etc., but also in extending the impact of silicon-based technology. Recently, electrolyte-gated hybrid devices incorporating redox-active molecules have been proposed. FIGS. 1A and 1B illustrate such a hybrid device. More particularly, as shown in FIG. 1A, the device 100 may include a substrate, such as a silicon substrate 101, a molecular layer 110, an electrolyte 120 directly on the molecular layer 110, and a conductive gate, such as a metal gate 130. The electrolyte 120, besides being a conducting medium between the gate 130 and the substrate 101, may also assist in the oxidation and reduction processes of the charge storage molecules in the molecular layer 110 by forming a double layer 115 at the electrolyte-molecule interface, as shown in FIG. 1B. This double layer 115 may be extremely thin (˜10 Å), and may behave like an insulator.
More specifically, as illustrated in the enlarged view of FIG. 1B, when a sufficient voltage is applied to the gate 130, the charge storage molecules 105 in the molecular layer 110 (which includes a linker 102 configured to couple the charge storage molecules 105 to the substrate 101) may be positively charged, and oppositely charged ions 117 in the electrolyte 120 across the molecule-electrolyte interface may balance these positive charges. As such, due to the accumulation of charges at the interface, an electrostatic equilibrium is established, resulting in a “double layer” 115 of separated charges. The induced double layer 115 may substantially inhibit leakage between the electrolyte 120 and the charge-storage molecules 105. Further description of electrolyte-gated hybrid devices can be found in U.S. Pat. No. 6,674,121 to Misra et. al and U.S. patent application Ser. No. 10/837,028 to Bocian et al., the contents of which are incorporated by reference in their entirety.